Scanning a bound book, or other non-planar objects, using a conventional document scanner can result in three image quality defects: geometric distortion, illumination variation, and defocus. The present disclosure relates to the art of digital imaging. It finds particular application in conjunction with estimating local defocus distance and correcting defocus blur and geometric distortion in a scanned image of a bound book, and will be described with particular reference thereto. It is to be appreciated, however, that the disclosure will also find application in conjunction with correction of defocus blur and geometric distortions in other three-dimensional objects on a variety of image acquisition and output devices, such as scanners, xerographic copiers and printers.
Optical systems in document scanners and scanning copiers, such as a Selfoc® lens systems and conventional optical systems, possess a depth of field over which the optical system produces an image that meets resolution and distortion specifications. Conventionally, when a book or other bound, multi-page original is scanned by the scanner, the resulting digital image contains geometric distortions and blurring in the region adjacent the non-planar areas. In other words, portions of certain non-planar original objects, such as books, wrinkled sheets, and three-dimensional objects, may lie outside the depth of field of the imaging system. For these originals, the distance between the original object and the ideal object plane, e.g. the scanner platen surface, varies from point to point which may cause a spatially varying blur in the acquired image. In addition to blurring, geometric distortion and illumination falloff often occur due to the varying defocus distance across the original being scanned.
One solution to the above problem is to modify the platen to have a sloping edge portion whereby the bound portion of the book, or other original object, is placed in a corner position such that the entire surface of the page being copied is in close contact with the platen surface. This system suffers from a limited magnification range because of restriction on scanned component movement near the sloping corner edge. In addition, operability and production are limited by the inability to perform a “split scan,” where both pages of a book are scanned without repositioning.
Another solution to the defocus blurring and geometric distortion problems employ an infrared distance sensor to determine the object defocus distance. In this method the plurality of sensed object defocus distances are used in a subsequent digital restoration algorithm. Another prior art technique uses one or more standard reference targets in order to determine the defocus or degraded states of specific scanner systems. The scanned image is then filtered with a fixed or time-varying image restoration filter, which utilizes correction coefficients pre-selected for the state of defocus or degradation. In addition, profile detectors have been inserted into the scanning system in order to measure the defocus distance of an object by using a reference image plane of the scanning system. The above techniques suffer from the disadvantage of additional required component costs as well as unwanted image processing complexity.